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The fate of phosphorus concerning its distribution in the thermal process and chemical speciation was studied during the co-combustion of sewage sludge with wheat straw and sunflower husks in powder combustion conditions. Co-combustion experiments were performed in a lab-scale entrained flow reactor (EFR) at 1000 °C and 1400 °C. SEM-EDS and ICP-OES analyses were used for studies of deposits collected on a probe, bottom ash, and particulate matter samples collected during experiments. Deposition probe samples were further studied and interpreted using powder X-ray diffraction (XRD) and thermochemical equilibrium calculations (TECs). The inorganic material in the different fuel particles mainly interacted through a molten phase observed on deposition probes. Crystalline P was mainly identified in β-Ca3(PO4)2 whitlockites. TECs support the experimental findings and suggest that a mostly homogenous melt occurs at 1400 °C, whereas Fe-oxides and Ca-phosphates precipitate during the cooling of the formed deposits. It was found that <5 % of incoming P was collected in fine particulate matter (<1 µm), indicating that the majority of P can be found in deposits and bottom ash. This outcome implies that P recovery efforts should be focused on these ash fractions.